Implementation of a Transformerless Common-Mode Active Filter for Offline Converter Systems

This paper presents a study and practical implementation of an active filter employing a high-frequency (HF) power amplifier and passive-filter components to be connected to the ac power lines in order to mitigate common-mode conducted emissions of three-phase pulsewidth modulation converter systems. The filter topology is chosen from different possibilities listed in a literature survey and studied regarding practical implementation issues, where requirements for an HF power amplifier to be applied in active filtering are derived. Special attention is put on the stability analysis where the challenges for the feedback are discussed, and a simple feedback structure is proposed. Other feedback concepts are analyzed, and limitations posed by stability requirements are presented. A prototype is designed and built, from which mathematical and experimental results are obtained demonstrating the potential and limitations of such a system.